Calendering is an economic and highly efficient means to produce film and sheet from plastics such as plasticized and rigid poly(vinyl chloride), abbreviated herein as “PVC”, and polypropylene compositions. The film and sheet usually have a thickness ranging from about 2 mils (0.05 mm) to about 80 mils (2.0 mm). Calendered PVC film or sheet are readily thermoformed into various shapes can be used in a wide range of applications including packaging, pool liners, graphic arts, transaction cards, security cards, veneers, wall coverings, book bindings, folders, floor tiles, and products which are printed, decorated, or laminated in a secondary operation. Additional discussion on polypropylene resin compositions used in calendering processes may be found in Japan Application No. Hei 7-197213 and European Patent Application No. 0 744 439 A1.
By contrast, conventional processing of polyesters into film or sheet involves extruding a polyester melt through a manifold of a flat die. Manual or automatic die lip adjustment is used to control thickness across a web of material. Water-cooled chill rolls are used to quench the molten web and impart a smooth surface finish. Although extrusion processes produce film and sheet of excellent quality, extrusion methods do not have the throughput and economic advantages of calendering processes.
PVC compositions are, by far, the largest segment of the calendered film and sheet business. Small amounts of other thermoplastic polymers such as, for example, thermoplastic rubbers, certain polyurethanes, talc-filled polypropylene, acrylonitrile/buta-diene/styrene terpolymers (ABS resins), and chlorinated polyethylene, are sometimes processed by calendering methods. By contrast, polyester polymers such as, for example, poly(ethylene terephthalate), abbreviated herein as “PET”, or poly(1,4-butylene terephthalate), abbreviated herein as “PBT”, are difficult to calender successfully. For example, PET polymers with inherent viscosity values of about 0.6 dL/g have insufficient melt strength to perform properly on the calendering rolls. Also, when the polyester is fed to the rolls at typical processing temperatures of 160° C. to 180° C., the PET polymer crystallizes causing a non-homogeneous mass which causes high forces on the calender bearings and is unsuitable for further processing. Another problem is the tendency of polyester polymers to hydrolyze during processing in the molten or semi-molten state on rolls exposed to atmospheric moisture. Typical PET polymers also have a tendency to stick to the calendering rolls at typical processing temperatures. Avoidance of these difficulties often requires a careful selection of polymer properties, additives, and processing conditions. The calendering of various polyester compositions has been described, for example, in U.S. Pat. Nos. 5,998,005; 6,068,910; 6,551,688; U.S. patent application Ser. No. 10/086,905; Japan Patent Application No.'s 8-283547; 2001-274010; 7-278418; 2000-243055; 10-363-908; 2000-310710; 2001-331315; 11-158358; and World Patent Application No. 02/28967.
Many current applications for calendered films have stringent flammability requirements. The calendered films prepared from polyesters typically do not show sufficient flame retardancy to be useful in many commercial applications. Further, the flame retardants often used with various polymer compositions often are not compatible with molten polyesters, are reactive with the polyester or its various additives, or do not impart the desired degree of flame retardancy at suitable concentrations.
To obtain film or sheet with desirable levels of flexibility, polymeric materials such as PVC and cellulose esters must be plasticized before calendering. Most other thermoplastic resins, however, such as polyesters, polyamides and polyolefins, do not typically contain a plasticizer when processed in the molten state to form rigid molded or extruded objects. Plasticizers increase the flexibility and softness of calendared polyester films, improve the processing of the polyester, and help to prevent sticking of the polyester to the calender rolls. The addition of plasticizers, however, generally increases the available fuel and results in increased flammability. The increased flammability of plasticized polyesters has created a need for flame-retardant, plasticized, polyester compositions suitable for the production of film and sheet by calendering as a more economical alternative to extrusion processes.